presence.js

Fixes Smack throwing exceptions (flashless version)
The problem was being caused by the PresenceServlet calling Smack in order to create a new room which already existed.

Example:
-shaun has 5 lessons, each with their chatrooms which are named using the lessonId ie 1,2,3...
-shaun gets rebuilt, no more lessons, lessons restart counting at 1
-jabber server already has a room named "1" so it throws an exception

The lessonId as chatRoomName is bound to cause problems if different servers are using the same chatServer. Therefore, room names are now formed by concatenating lessonId to createDateTime and by replacing " " and ":" characters with underscores (Jabber servers don't accept those characters).